This invention relates generally to underwater acoustic signal generators, and, more particularly, relates to undersea apparatus and methods for generating acoustic signals with high source level, efficiency and reliability.
The advent of new oceanographic research fields such as acoustic tomography, the expansion of existing fields such as marine geology and mineral resource exploration, and the development of new applications such as a proposed global ocean warming monitoring system and an ultra-distant early warning submarine detection sonar system, require the generation of powerful and reliable low frequency underwater sound radiators that operate in the frequency range below one thousand hertz.
In order to be effective, such radiators must function reliably for years without preventive maintenance, and must provide a high source level and broadband frequency performance, and must be submersible to significant ocean depths, on the order of one thousand meters. Combining all of these requirements in one radiator in the frequency range below 1,000 Hz presents a set of difficult and often conflicting tasks.
The following patents and publications provide examples of undersea acoustic radiators:
______________________________________ U.S. Pat. No. Patentee ______________________________________ 1,791,014 Settegast et al. 3,143,999 Bouyoucos 3,277,437 Bouyoucos 3,403,374 Mellen et. al. 3,610,366 Goldberg ______________________________________ J. E. Blue et al., "A LowFrequency, TowPowered Sound Source", Power Transducers for Sonics and Ultrasonics, Proceedings of the International Workshop held in Toulon, France, June 12 and 13, 1990, B. F. Hamonic et al., ed., SpringerVerlag, pub., pp. 178-185. F. Massa, "Sonar Transducers: A History", Sea Technology, November 1989. O. B. Wilson, Introduction to Theory and Design of Sonar Transducers, 1988, Chapters 6 and 10 (pp. 109-125 and 171-182).
The conventional undersea acoustic radiators discussed in the above-listed publications, however, suffer from a number of deficiencies. One of the major drawbacks is low reliability associated with fatigue failures due to large deformations of the radiation sources, or excess friction from seal elements employed between a piston and a chamber.
In particular, the above-listed publications show that high acoustic power output for a small (compared to the sound wavelength) radiator can be achieved using a resonant monopole-type transducer at a large volume velocity, with large deformation of the radiating surface of the transducer. However, this characteristic is antithetical to the requirement of high reliability, in that large deformations lead to fatigue failures.
Moreover, the requirement of resonant transducer design, necessitated by high source levels, contradicts the desired characteristic of broadband frequency performance. As frequency decreases, it becomes even more difficult to satisfy these contradictory requirements using traditional reversible types of transducers, such as the electromagnetic, magnetorestrictive, and piezoelectric transducers.
Accordingly, the development of transducers that can satisfy these conflicting requirements at low frequencies is a longstanding and challenging problem.
It is accordingly an object of the invention to provide improved low frequency underwater acoustic radiation methods and apparatus.
It is another object of the invention to provide undersea low frequency acoustic radiation methods and apparatus affording high source level and high reliability.
It is a further object of the invention to provide acoustic radiation methods and apparatus that enable control and change of resonant frequency to attain broadband frequency performance.
Other general and specific objects of the invention will in part be obvious and will in part appear hereinafter.